In recent years, according to high capacity and high output trends of a secondary battery, there is a growing demand for high strength, high permeability and thermal stability of a separator, and improved properties of a separator for electrical safety of a secondary battery during charging and discharging. The lithium secondary battery is required to have high mechanical strength for improving safety in a battery manufacture process and during use of the battery, and to have high permeability and high thermal stability for improving a capacity and output.
For securing safety and improving the lithium secondary battery, among various constituents included in the lithium secondary battery, physical and electrochemical safety of the separator is particularly important. For example, when thermal stability of the separator is lowered, an inter-electrode short circuit due to damage or deformation of the separator caused by a temperature rise in the battery may occur, thereby increasing a risk of overheating or fire of the battery. In addition, as an application scope of the lithium secondary battery is extended to a hybrid vehicle, and the like, securing safety of the secondary battery following overcharge has become an important requirement, and a property of the separator which may withstand electrical pressure following overcharge is required.
Thermal stability of the secondary battery is influenced by a closing temperature, a melt fracture temperature, thermal shrinkage, or the like of the separator. Among them, thermal shrinkage at a high temperature has a great influence on the thermal stability of the battery. In the case that the thermal shrinkage is high, when the inside of the battery reaches high temperature, an edge portion of an electrode is exposed during shrinkage to cause the inter-electrode short circuit, which leads to heat/ignition/explosion, or the like. In addition, even in the case that a melt fracture temperature of the separator is high, but the thermal shrinkage is how, an edge portion of the battery is exposed while the electrode is heated, thereby causing the inter-electrode short circuit.
In order to overcome the safety problem of an electrochemical device as described above, as a porous polymer substrate of a generally used separator, polyolefin-based polymer films such as polyethylene (PE) and polypropylene (PP) films are widely used, which are advantageous for pore formation and has excellent chemical resistance, mechanical physical properties and thermal properties. However, polyolefin has severe thermal shrinkage at a nigh temperature, and is physically vulnerable. Usually, a method of forming a polyolefin-based microporous film, and then laminating an inorganic layer on the surface thereof is adopted, as a method of raising thermal resistance of a polyolefin film. However, during forming the polyolefin-based microporous film, heat setting is performed, and in this process, the film is partially melted, so that the surface pores of the porous film are closed and damaged, thereby greatly increasing a ventilation time.
In addition, since a conventional separator including an inorganic layer uses an organic solvent in the manufacturing process, there are some problems in a coating method based on a binder composition dissolved in the organic solvent. First, when the organic solvent is volatilized in the drying process, a binder dissolved in the organic solvent forms a gel, thereby generating a solvent-impermeable space, resulting in an unbalanced organic/inorganic coating layer, and this phenomenon may lower battery characteristics. In addition, explosion proof equipment is needed, and byproducts harmful to the environment or a human body occur in the working process. Also, there is a problem in that in the state that the binders are dissolved in the organic solvent, pores of the porous polymer substrate are closed.
In order to solve the problem, Korean Patent Laid-Open Publication No. 10-2016-0041492 suggests a method of using a polyvinylidene fluoride dispersion and aqueous slurry including inorganic particles and organic particles to form a coating layer on a porous polymer substrate. As the separator therefor, a separator for an electrochemical device improving adhesive strength with a porous substrate to have excellent thermal resistance and strength has been suggested, however, a degree of thermal and electrochemical safety of the separator is still insufficient to be used for securing safety of the battery, whereby improvement of a battery capacity is still needed.
A process for manufacturing a novel separator having excellent thermal and electrochemical stability, and being advantageous for securing pores, for solving the problems is demanded.